/Engine/Plugins/Experimental/ScriptableToolsEditorMode/Source/ScriptableToolsEditorMode/Public/ScriptableToolsEditorModeStyle.h
FileNameWithPath must be the full on-disk path to the icon, which is constructed in code from a relative path the user enters as (eg) a property/etc. To help with debugging, this path can be provided as ExternalRelativePath, and it will be printed out instead of the absolute path as part of the log error message.
static bool TryRegisterCustomIcon ( [FName](API\Runtime\Core\UObject\FName) StyleIconIdentifier, [FString](API\Runtime\Core\Containers\FString) FileNameWithPath, [FString](API\Runtime\Core\Containers\FString) ExternalRelativePath )